1. Field of the Invention
This invention relates generally to information recording and reproducing apparatus and, more particularly, is directed to a tracking control system for an information recording and reproducing apparatus.
2. Description of the Prior Art
Generally, in an information signal recording and reproducing apparatus, such as a video tape recorder (VTR), the quality of the reproduced signal deteriorates if there is a deviation or tracking error between the position of a recorded track on a magnetic recording medium and the position of a reproducing magnetic head as the latter traces the recording medium during the reproduction operation. In order to eliminate this tracking error, it is necessary to effect a tracking control operation. For example, in a VTR where the video signal is recorded and reproduced by means of a rotary magnetic head assembly, the aforementioned tracking control operation can be performed by a capstan servo-control operation with respect to a capstan motor, the latter of which controls the speed of advancement of the magnetic recording medium in accordance with the tracking error. Another example of a tracking control system that has been used with conventional VTRs is described in U.S. Pat. No. 4,151,570 to Ravizza. In this patent, a wobbling or dithering signal is used to provide a slight vibration to the reproducing magnetic head, whereby the position of the reproducing magnetic head is controlled such that the envelope of the reproduced signal therefrom is at a maximum. However, with the capstan servo-control system, above-described, accurate reproduction of a track from the beginning to the end thereof may not be possible, as discussed more fully in copending U.S. patent application Ser. No. 06/261,445, filed May 7, 1981, having a common assignee herewith, and the disclosure of which is incorporated herein by reference. It is to be appreciated that, since accurate tracking cannot be obtained for the entire length of a track using the aforementioned capstan servo-control system, this system has serious drawbacks, especially when used with respect to narrow recording tracks. With the tracking control operation effected by use of a wobbling or dithering signal, although an increased response speed is obtained over the capstan servo-control system, the effective level of the envelope of the reproduced signal is reduced to about 90% by such wobbling, whereby deterioration of the overall signal-to-noise (S/N) ratio inevitably results.
Accordingly, it has been proposed in the aforementioned U.S. patent application Ser. No. 06/261,445, to digitize the video signal and then separate the digitized video signal into two separate channels prior to recording the signal on a magnetic tape. A magnetic head is associated with each channel and the two magnetic heads are aligned to record the respective channels on a magnetic tape in parallel tracks extending obliquely on the tape while also providing the two magnetic heads with different azimuth angles. During a reproduction operation, a tracking control operation is effected by obtaining a tracking error in accordance with the phase difference between the reproduced signals from the individual channels. In this manner, steady and accurate tracking control is obtained without deterioration of the S/N ratio and with a high response speed.
It is to be appreciated that the recording of a digitized color video signal in two channels requires the use of a high bit rate for the recording signal, whereby a large quantity of magnetic tape is used. To overcome this deficiency, it has been proposed to further distribute the recording signal to at least four channels by use of a corresponding number of magnetic heads, thereby reducing the bit rate of the recording signal. In such case, four rotary magnetic heads are used to record the signal in four parallel and adjacent channel tracks for each field of information, with the first and third alternate tracks having the same first azimuth angle and the second and fourth alternate tracks having a second azimuth angle equal and opposite to the first azimuth angle. Since the phase difference between the signals in the first and second tracks or the phase difference between the signals in the third and fourth tracks can be used to effect a tracking control operation, two stable operating positions for the rotary heads exist. In particular, a first stable operating position exists when the first through fourth heads scan the first through fourth tracks in a correct and desired manner. The second operating position occurs, when the first, second, third and fourth heads scan the third, fourth, first and second tracks, respectively. This latter operating position causes a phase shift in the reproduced signals which of course, is undesirable. It is therefore desirable that the tracking control system operate in a stable manner only when the heads scan the tracks in the aforementioned first operating position.